Optical evaluation of a planar lightwave circuit (PLC) is conducted by connecting an optical fiber to an optical waveguide of the planar lightwave circuit and causing light to enter the optical waveguide from the outside via this optical fiber. In this case, conventionally, a refractive index of the optical waveguide is adjusted to that of the optical fiber, and an end surface of the optical waveguide is directly connected to a flat end surface of the optical fiber. Although a gap occurs between the end surface of the optical waveguide and the end surface of the optical fiber due to roughness of the connection surface, scattering of light from a boundary between the optical waveguide and the optical fiber is prevented by providing matching oil between the end surfaces.
By the way, along with development of silicon photonics, a silicon wire waveguide in which silicon is used as a core and silicon dioxide is used as a clad has come to be used in a PLC, as described in Patent Literature 1 (PTL1). Since a minimum bend radius can be made small in a silicon wire waveguide, a large number of optical integrated circuit chips each having an optical waveguide can be disposed on a single silicon wafer.
However, in conventional optical evaluation of a PLC, it is necessary to cut an end surface of an optical waveguide into chips by a method such as dicing and conduct measurement for each measured portion by bringing an end surface of an optical fiber into contact with the end surface of the optical waveguide.
Since the width of a silicon wire waveguide is approximately 1/10 of that of an optical fiber, a large refractive index difference exists between the optical fiber and the silicon wire waveguide. Therefore, a lensed fiber, i.e., an optical fiber whose tip has a lens shape, is also used in order to surely achieve optical coupling between the optical fiber and the silicon wire waveguide.
This, however, necessitates a troublesome process of aligning the optical fiber and the silicon wire waveguide with high alignment accuracy.
In view of this, PTL 1 and Non-Patent Literature 1 (NPL1) disclose an arrangement in which a diffraction grating is formed within a plane of a silicon wafer, and optical evaluation of an optical circuit is conducted by causing an optical fiber to input/output light to/from this diffraction grating. More specifically, the optical fiber is provided so as to face the diffraction grating at an inclination of approximately 10° with respect to a direction perpendicular to a surface of the silicon wafer, and light is input/output to/from the tip of the optical fiber from/to the diffraction grating.